Mercury in Bays and Ponds of Cape Cod

In recent work conducted in Waquoit Bay, we found highly elevated
concentrations of dissolved total mercury (Hg). Simultaneous
investigation of groundwater found similarly high levels which
suggested submarine groundwater discharge as a significant source to
that embayment. This was a new finding, which has garnered scientific
attention (cover article in ES&T along with commentary/news
release) and local concern (front page article in the Cape Cod Times,
March 22 and 24 and a story on NPR). Our work comes on the heels of
consumption advisories for fish in some Cape ponds (Ashumet and most
recently, several ponds within the National Seashore boundaries). In
all these cases, the primary source appears to be atmospheric
deposition as it is in most locations in the U.S. The situation of Cape
Cod may be unique in that our highly permeable and low organic carbon
soils are poor at holding back and storing the Hg loaded onto our
watersheds. The result appears to be that more Hg finds its way into
our aquatic foodwebs than in locations with similar atmospheric
loading.

At the moment, however, this remains a hypothesis. A first important
test is to begin to assess the scope of the problem by examining a
number of water bodies on the Cape. This would include fresh and salt
waters as both receive groundwater inputs. We will measure the amount
of mercury and a bioaccumulating microbial metabolic byproduct form,
monomethylmercury, in local pond and embayment waters, sediments and
organisms. While doing this we will also measure the amount of
dissolved radon (Rn). This will allow us to assess whether systems that
receive a lot of groundwater (as indicated by Rn) also have high levels
of Hg, and the chemical form of the Hg entering. Finally, we will
explore the use of sediment cores and flux chambers for determining
that amount of monomethylHg (the bioaccumulating form of Hg) that
diffuses from bay sediments into the overlying water, and thereby begin
to determine the relative importance of groundwater versus coastal
production as a source of this form of Hg.

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